1. Field of the Invention
The present invention relates generally to a spread-spectrum receiver for a communication system of code-division multiple access scheme (hereinafter also referred to as the CDMA scheme or CDMA communication system) which is used in mobile communication systems such as, e.g. for mobile digital-cellular phone applications. More particularly, the present invention is concerned with a spread-spectrum receiver which is so arranged as to control delay times of timing adjusting buffers by making use of timing signals corresponding to differences in timings among paths for thereby making it possible to take in a RAKE combining the signal on the path(s) delayed one symbol period or more as well.
2. Description of Related Art
According to the CDMA scheme adopted in the mobile communications such as for mobile digital-cellular phone communication and the like, a same frequency band can be used by a plurality of channels at a same time. To this end, at the sender side, transmission symbol data are multiplied by spreading codes which differ from one to another channel to thereby generate a transmission signal to be sent out. In this conjunction, the processing for multiplying the transmission symbol data by the spreading codes, as mentioned above, is referred to as "spreading processing", while the transmission signal (i.e., the signal for transmission) as generated is referred to as "spread spectrum signal". As the spreading codes, there are employed a series of codes generated at high rate on the order of ten and several times to several hundred times as high as the rate of the transmission symbol data. Parenthetically, a minimum unit or element of the spreading code is referred to as a "chip".
The spread-spectrum receiver for the CDMA communication system is adapted to receive the transmission signal sent from a base station. In the spread-spectrum receiver, the received signal is multiplied with a replica code (also referred to as "despreading code" and actually the same as the spreading code used at the sender side) allocated to the receiver at a synchronized timing, whereon the signal resulting from the multiplication is integrated on a symbol-by-symbol basis. This processing is referred to as "correlation processing" or "despreading processing", while the integration value as obtained is referred to as "correlation value". Incidentally, the circuit for performing the correlation processing is called a "correlator". In the correlation processing, when the replica code used at the receiver is same as the spreading code employed at the sender, a large correlation value can be obtained. In that case, the correlation value as obtained bears equality to the transmission symbol data. On the contrary, when the replica code used at the receiver differs from the spreading code used at the sender, it is impossible to derive the correlation value. Accordingly, upon reception of multiple transmission signals resulting from simple addition of the transmitting signals of a plurality of channels, it is possible to derive only the transmission symbol data of the channel of concern by performing the correlation processing on the multiple received signal and the spreading code allocated to the channel of concern. This is the principle underlying the demodulation of the spread spectrum signals separately for the individual channels, respectively, in the CDMA communication system.
In order to demodulate the spread spectrum signal, the timing of the spreading code contained in the received signal has to coincide with the timing of the replica code used in the receiver. Deviation of the timing of the replica code from that of the spreading code even by only one chip incurs the same result as that of the multiplication of the received signal by utterly irrelevant replica code, making it impossible to obtain a large correlation value. For this reason, in the CDMA communication system, timing synchronization of extremely high accuracy is required between the spreading code and the replica code.
One of the major causes for the deterioration or lowering of the frequency utilization efficiency in the CDMA communication system is interference by other users' apparatuses. When the transmission power is lowered at all the users' apparatuses, mutual interference can certainly be mitigated or suppressed. However, the communication quality at the individual users' apparatuses will become degraded. Under these circumstances, there arises a demand for technology which can ensure high communication quality even at a low reception power level. Although there have been proposed and developed numerous techniques for ensuring the high communication quality, they can not exhibit the intrinsic capabilities unless the synchronization of the spreading code with the replica code is established. Thus, it is safe to say that the synchronization state of the spreading code governs the performance of the CDMA communication.
At this juncture, it should be mentioned that overland mobile communication involves multiple propagation paths with different delays due to reflections and diffractions of radio waves at buildings, mountains and the like. In the conventional narrow-band communication system known heretofore, the radio wave of the temporally preceding symbol exerts interference to the temporally succeeding symbol radio wave (known as the inter-symbol interference), bringing about remarkable degradation in the characteristics. For suppressing or preventing such inter-symbol interference, it is required to use an adaptive equalizer and the like, which however involves much complicated and expensive system configuration, to a great disadvantage.
By contrast, in the case of the spread-spectrum communication system, the timing of the replica code can match with the signal of only one of the multiple paths, if any, while the timing of the replica code is out of match with the signals on the other paths. Thus, only the signal on the one path of concern makes appearance in the signal obtained after the demodulation, avoiding essentially any influence from the signals on the other paths. As is apparent from the foregoing, the spread-spectrum communication system can ensure high temporal resolution power, making it unnecessary to provide the adaptive equalizer, because of very low probability of occurrence of inter-symbol interference notwithstanding the presence of multiple paths.
For demodulating the received signals from the multiple paths, it is required first to measure or determine the timing of the signal from each of the multiple paths independently. This measurement can be realized by performing the correlation processing for the received signal and the replica code while shifting the timing of the replica code. By plotting then the correlation values with the timing of the replica codes being taken along the abscissa while the correlation values at the individual timings are taken along the ordinate, a graph referred to as a "delay profile" can be obtained (see FIGS. 4B and 4C). At this juncture, it should be mentioned that the values T taken along the abscissa does not indicate the real time but indicate "shift of the replica code".
In the delay profile, the signals on the individual paths are represented by independent or discrete pulse waveforms, respectively. Thus, the delay profile substantially corresponds to the impulse responses of the propagation paths. For these reasons, a plurality of replica code generators and a corresponding number of correlators are employed, wherein an arrangement is made such that the timings of the individual replica codes can be set independent of one another. The timings of plural replica codes are then caused to match with the timings on the corresponding number of the paths, respectively, as read out from the delay profile. In this manner, the signals on the individual paths can be demodulated independent of one another in the state bringing about no mutual interference. Since these plural demodulated signals carry the same information, there can be realized enhanced reception quality by synthesizing the demodulated signals. This is known as the path diversity effect. The procedure mentioned above is called the "RAKE scheme" because of resemblance to the function of a rake in that the signals are collected together from the individual paths. The RAKE scheme or function intrinsically owes to the spread-spectrum communication system, taking advantage of obstacles encountered in the multiple propagation environment, and thus the RAKE scheme is indispensable for ensuring high quality for the communication in the spread-spectrum communication system.
In this conjunction, a set of the replica code generator, the correlator and the synchronous detector employed for demodulating the signal on one path is referred to as a "finger" by likening to the finger of the rake. Accordingly, for realizing the demodulation and RAKE combination of the signals on n paths, there are required n fingers, i.e., n sets each composed of the replica code generator, the correlator and the synchronous detector. Parenthetically, the matching of the timing of the replica code to the timing of the signal on a given path is referred to as "finger allocation".
In order to make the most of the advantageous feature of the RAKE scheme, it is necessary to determine the timing of the signal on each path as well as the reception level thereof for thereby realizing the synchronization reliably without fail by allocating the timings of the signal on the individual paths to the fingers sequentially in the order of high to low reception levels. This sequence is referred to as "search". By performing the search processing, synchronization can be established in the spread-spectrum receiver of the CDMA communication system.
For having better understanding of the underlying concept of the invention, the technical background thereof will reviewed in some detail. FIG. 1 of the accompanying drawings is a block diagram showing generally a structure of a conventional spread-spectrum receiver according to a three-finger RAKE scheme. Referring to the figure, a high frequency signal received by an antenna 1 undergoes frequency conversion and quadrature detection in a radio circuit 2 to be converted into a base-band signal, which is then inputted to a timing control circuit 3 for determining timings for first to third paths, respectively, through the synchronous search processing. The first timing signal corresponding to the first path is inputted to a first replica code generator 4.sub.1 and a first correlator 5.sub.1, the second timing signal corresponding to the second path is inputted to a second replica code generator 4.sub.2 and a second correlator 5.sub.2, and the third timing signal corresponding to the third path is inputted to a third replica code generator 4.sub.3 and a third correlator 5.sub.3. In the first to third replica code generators 4.sub.1 to 4.sub.3, codes, each of which is the same as the spreading code used in the spreading process performed at the sender side, are generated as first to third replica codes (also referred to as the reverse-spreading codes), respectively. The first to third replica codes are supplied to the first to third correlators 5.sub.1 to 5.sub.3 from the first to third replica code generators 4.sub.1 to 4.sub.3, respectively, at the timings given by the first to third timing signals, respectively. In the first correlator 5.sub.1 the base-band signal inputted from the radio circuit 2 is multiplied by the first replica code, whereupon the product signal resulted from the multiplication is integrated to obtain a first correlation value on a symbol-by-symbol basis. The first correlation value undergoes synchronous detection in a first synchronous detector 6.sub.1 to be thereby converted into a first symbol signal. The first symbol signal is then latched by a first latch circuit 7.sub.1 serving as a timing adjusting buffer in response to a symbol timing pulse supplied from the timing control circuit 3. In the second correlator 5.sub.2, the base-band signal is multiplied by the second replica code, whereupon the product signal resulted from the multiplication is integrated to obtain a second correlation value on a symbol-by-symbol basis. The second correlation value undergoes synchronous detection in a second synchronous detector 6.sub.2 to be thereby converted into a second symbol signal. The second symbol signal is latched by a second latch circuit 7.sub.2 serving as a timing adjusting buffer in response to the symbol timing pulse. In the third correlator 5.sub.3, the base-band signal is multiplied by the third replica code, whereupon the product signal resulted from the multiplication is integrated to obtain a third correlation value on a symbol-by-symbol basis. The third correlation value undergoes synchronous detection in a third synchronous detector 6.sub.3 to be thereby converted into a third symbol signal. The third symbol signal is latched by a third latch circuit 7.sub.3 serving as a timing adjusting buffer in response to the symbol timing pulse. The output signals of the first to third latch circuits 7.sub.1 to 7.sub.3 are synthesized or combined together by a RAKE combining circuit 8 to be outputted therefrom as a demodulated signal.
As can be seen in FIG. 2, symbol signals a1, b1 and c1 are outputted sequentially from the first finger (composed of the first replica code generator 4.sub.1, the first correlator 5.sub.1 and the first synchronous detector 6.sub.1), symbol signals a2, b2 and c2 are outputted sequentially from the second finger (composed of the second replica code generator 4.sub.2, the second correlator 5.sub.2 and the second synchronous detector 6.sub.2) at timings delayed a little relative to the symbol signals a1, b1 and c1, respectively, and symbol signals a3, b3 and c3 are outputted sequentially from the third finger (composed of the third replica code generator 4.sub.3, the third correlator 5.sub.3 and the third synchronous detector 6.sub.3) at timings delayed a little relative to the symbol signals a2, b2 and c2, respectively. Accordingly, by generating sequentially symbol timing pulses ta, tb and tc from the timing control circuit 3 immediately before the boundaries of every one symbol period in the most leading path, to thereby allow the output signals of the first to third fingers to be latched by the first to third latch circuits 7.sub.1 to 7.sub.3 in response to the symbol timing pulses ta, tb and tc, respectively, then the symbol signals a1, a2 and a3 are latched by the first to third latch circuits 7.sub.1 to 7.sub.3, respectively, at the same timing in response to the symbol timing pulse ta, the symbol signals b1, b2 and b3 are latched by the first to third latch circuits 7.sub.1 to 7.sub.3, respectively, at the same timing in response to the symbol timing pulse tb, and the symbol signals c1, c2 and c3 are latched by the first to third latch circuits 7.sub.1 to 7.sub.3, respectively, at the same timing in response to the symbol timing pulse tc. Because the corresponding symbol signals are latched by the first to third latch circuits 7.sub.1 to 7.sub.3, respectively, in this way, there can be derived the demodulated signals a, b and c by combining or synthesizing the output signals of the first to third latch circuits 7.sub.1 to 7.sub.3 by the RAKE combining circuit 8.
The conventional spread-spectrum receiver of the CDMA communication system, however, suffers a shortcoming in that the symbol signal on the path presenting a delay corresponding to one symbol period or more relative to the path leading topmost in the timing cannot be latched at the same time with the signals on the other paths because there is provided only one stage of latch circuit serving as the timing adjusting buffer between the finger and the RAKE combining circuit. To say this in another way, only the signals on the paths presenting the delay less than one symbol period can be synthesized by the RAKE combining circuit, which in turn means that the pulses susceptible to the RAKE combination are limited. Consequently, the reception level of the spread-spectrum receiver can not effectively be increased even when the number of the fingers is increased, providing this difficulty in improving the communication quality of the CDMA communication system.